1. Field of the Invention
The invention relates to a method and an apparatus for concentrating aqueous alkali in plants for producing 1,2-dichloroethane.
2. Description of the Related Art
1,2-Dichloroethane, (hereinafter “DCE”) is used predominantly as intermediate in the preparation of monomeric vinyl chloride which is in turn used as intermediate for the preparation of polyvinyl chloride. The reaction of DCE to form monomeric vinyl chloride forms hydrogen chloride HCl. This is preferably used in the preparation of DCE by oxychlorination of ethene by means of HCl and oxygen. An alternative route for preparing DCE proceeds via the direct chlorination of ethene by means of chlorine. In the industrial preparation of DCE, both routes are employed, so that there is a balance between the hydrogen chloride produced and that which is consumed, according to the following reaction equations:Cl2+C2H4→C2H4Cl2+218 kJ/molC2H4Cl2→C2H3Cl+HCl−71 kJ/molC2H4+2HCl+½O2→C2H4Cl2+H2O+238 kJ/mol
The chlorine required for the direct chlorination is frequently produced in a chloralkali electrolysis plant located in the vicinity of the DCE plant. As is known, sodium hydroxide solution is also formed here, typically in a concentration of about 30% by weight.
This alkali represents a material of value but has to be concentrated for economic reasons. Modern chloralkali electrolysis plants are equipped with a concentration unit for the alkali obtained. This unit is typically operated by means of steam and typically produces alkalis having a concentration of 50% by weight or more of sodium hydroxide. The amount of steam required for operating the concentration unit represents a not inconsiderable part of the operating costs of the chloralkali electrolysis plant.
It has already been proposed that heat from the DCE process be used for evaporating or concentrating sodium hydroxide solution from the chloralkali electrolysis. DE 10 2005 044 177 A1 describes a method and an apparatus for utilizing the heat of condensation obtained in the purification of DCE from a direct chlorination plant. Here, the heat of condensation of the vapor obtained in the purification of DCE by distillation is at least partly used for evaporation of the sodium hydroxide solution originating from a chloralkali electrolysis plant. DE 10 2005 030 511 A1 and DE 10 2005 030 512 A1 describe a method and apparatuses for utilizing the heat of reaction arising in the preparation of DCE. Here, the heat of reaction from a direct chlorination plant is at least partly used for evaporation of the sodium hydroxide solution originating from a chloralkali electrolysis plant. Apparatuses which comprise a shell-and-tube heat exchanger having two fixed tube plates and an NaOH bottom part and are configured so that sodium hydroxide solution is conveyed inside the tubes and DCE is conveyed along the outside of the tubes and have devices for introducing sodium hydroxide solution into the interior of the tubes and distributing it over them are used.
These previously known methods and apparatuses allow very efficient operation of the process. For the purposes of process optimization, further process improvements are sought and the process should be able to be operated in an even more resource-conserving manner.